1. Field of the Invention
The present invention relates to an image forming apparatus for reading an image from a film-type original document such as a color negative film or a color positive film and/or from a reflection-type original document to print a copy on which an image is formed through various image processings.
2. Related Background Art
Shading correction data has heretofore been obtained by illuminating a white plate by means of an exposure lamp or by introducing the light from the exposure lamp directly into a sensor. However, in the case of a color negative film, the density value of its orange base corresponds to a highlight value. Therefore, no density values within a range between the quantity of illumination light and the density value of the orange base are employed in signal processing. As a result, the dynamic range of a read signal is reduced and it is therefore impossible to obtain signals which can realize a high-gradation image.
To eliminate such disadvantage, the U.S. Pat. application Ser. No. 057,657, now U.S. Pat. No. 4,833,532, of the present applicant proposes that, in the case of a negative film, the gamma conversion characteristics of the film is modified to nullify the density of the orange base thereof.
However, even if such a gamma conversion is uniformly performed in accordance with the kind of film, the specific characteristics of the film make it difficult to perform an optimum control.
The U.S. Pat. application Ser. No. 013,119, filed on Feb. 20, 1987 by the present applicant, proposes a technique of reading the density of the orange base of a film to control the quantity of light or the gain thereof. However, there continues to be a problem in that it is impossible to correct the distribution of illuminance and the non-uniformity of sensitivity due to a difference in position between sensors.